The invention relates generally to processes for fabricating semiconductor devices, and deals more particularly with low cost processes for fabricating rectifiers and other low and high voltage semiconductor devices.
The following process was previously known to fabricate semiconductor rectifiers:
First, a silcon wafer is doped by diffusion to provide a p-n layer suitable for the intended application.
Second, the wafer is cut or sandblasted to provide individual "chips", also known in the art as "dice". Each of the chips contains a portion of the p-n layer forming a small p-n junction. The cutting operation may be performed by a disc saw having a diamond blade which is adapted to cut approximately two-thirds the way through the thickness of the wafer. Then, the wafer is fractured along the lines of cut into the individual chips. The sandblasting operation may begin by using wax to bond stainless steel resist elements in the shape of the desired silicon chips onto the surface of the wafer. Then a blast of sand wears channels through the wafer around the edges of the resist elements to divide the wafer into the desired chips. Both the fracturing step of the saw cut operation and the sandblast step of the sandblasting operation leave the edges of the silicon chips rough.
Third, silver-plated copper discs are soldered with silver solder onto the opposite faces of the individual silicon chips to provide electrodes.
Fourth, the electroded chips are subject to a chemical etching process to smooth the sides of the chips which were made rough by either the cutting or sandblasting operations. In the chemical etch step, the individual, electroded silicon chips are loaded into a beaker containing a mixture of hydrofluoric acid, nitric acid, and acetic acid, and the beaker is agitated. Then the electroded chips are rinsed in hydrogen peroxide, and later subject to additional chemical baths and rinses of the same types.
Fifth, each individual electroded chip is inserted into an individual mold, and a silicon resin, such as Silastic (trademark of Dow Chemical) resin, is poured into the mold around the sides of the electroded chip to passivate the exposed edges of the silicon chip.
Sixth, the resin is cured by heat.
There are several problems with the foregoing process. During the third through sixth steps, the silicon chips or electroded chips, as the case may be, are separate small pieces which must be handled individually. This is cumbersome and expensive, particularly during the fifth step when each of the chips is individually inserted into a mold. Also, there are frequently errors in the handling such that the polarity of the cells is reversed, and corresponding errors result in the subsequent utilization of the chip. Moreover, it is cumbersome to electrically test the chips because they are separated from each other and must be handled individually.
Further, the chemical etching steps of the foregoing prior art process require multiple baths, resulting in silver contamination from the electrodes and the solder onto the silicon chip. While the contamination is less severe than would be the case if the electrodes were not silver plated, such silver contamination degrades the reverse leakage current and high temperature performance of the finished product. Also, the requirement for a silver plate on each copper electrode is expensive.
Accordingly, a general object of the present invention is to provide a low cost method for fabricating passivated semiconductor devices such as rectifiers.
A more specific object of the present invention is to provide a process which does not require individual handling of a batch of silicon chips used to make the semiconductor devices.
Another specific object of the present invention is to provide processes which do not require silver plated electrode discs.
Still another specific object of the present invention is to provide processes which avoid metal contamination of the silicon chips.
Yet another specific object of the present invention is to provide processes which are adapted to the production of transistors, thyristors, MOSFETS and other high voltage semiconductor devices.
Yet another specific object of the present invention is to provide a batch of electroded silicon chips in a form convenient for electrical testing.